Neurotensin excitation of serotonergic neurons in the dorsal raphe nucleus of the rat in vitro

The dorsal raphe nucleus in the control of energy balance

Energy balance is orchestrated by an extended network of highly interconnected nuclei across the central nervous system. While much is known about the hypothalamic circuits regulating energy homeostasis, the 'extra-hypothalamic' circuits involved are relatively poorly understood. In this review, we focus on the brainstem's dorsal raphe nucleus (DRN), integrating decades of research linking this structure to the physiologic and behavioral responses that maintain proper energy stores. DRN neurons sense and respond to interoceptive and exteroceptive cues related to energy imbalance and in turn induce appropriate alterations in energy intake and expenditure. The DRN is also molecularly differentiable, with different populations playing distinct and often opposing roles in controlling energy balance. These populations are integrated into the extended circuit known to regulate energy balance. Overall, this review summarizes the key evidence demonstrating an important role for the DRN in regulating energy balance.

Antinociceptive role of neurotensin receptor 1 in rats with chemotherapy-induced peripheral neuropathy

Background

Chemotherapy-induced peripheral neuropathy (CIPN) is a major side effect of anti-cancer drugs. Neurotensin receptors (NTSRs) are widely distributed within the pain circuits in the central nervous system. The purpose of this study was to determine the role of NTSR1 by examining the effects of an NTSR1 agonist in rats with CIPN and investigate the contribution of spinal serotonin receptors to the antinociceptive effect.

Methods

Sprague–Dawley rats (weight 150–180 g) were used in this study. CIPN was induced by injecting cisplatin (2 mg/kg) once a day for 4 days. Intrathecal catheters were placed into the subarachnoid space of the CIPN rats. The antiallodynic effects of intrathecally or intraperitoneally administered PD 149163, an NTSR1 agonist, were evaluated. Furthermore, the levels of serotonin in the spinal cord were measured by high-performance liquid chromatography.

Results

Intrathecal or intraperitoneal PD 149163 increased the paw withdrawal threshold in CIPN rats. Intrathecal administration of the NTSR1 antagonist SR 48692 suppressed the antinociceptive effect of PD 149163 given via the intrathecal route, but not the antinociceptive effect of intraperitoneally administered PD 149163. Intrathecal administration of dihydroergocristine, a serotonin receptor antagonist, suppressed the antinociceptive effect of intrathecally administered, but not intraperitoneally administered, PD 149163. Injecting cisplatin diminished the serotonin level in the spinal cord, but intrathecal or intraperitoneal administration of PD 149163 did not affect this reduction.

Conclusions

NTSR1 played a critical role in modulating CIPN-related pain. Therefore, NTSR1 agonists may be useful therapeutic agents to treat CIPN. In addition, spinal serotonin receptors may be indirectly involved in the effect of NTSR1 agonist.

Neurotensin induces hypothermia by activating both neuronal neurotensin receptor 1 and astrocytic neurotensin receptor 2 in the median preoptic nucleus

Neurotensin (NTS) is a neuropeptide acting as a neuromodulator in the brain and is a very potent hypothermic agent. However, the cellular mechanisms of actions are not fully understood. Here we report that NTS increases the firing rate of preoptic GABAergic neurons by activating both neurotensin receptor 1 (NTSR1) and neurotensin receptor 2 (NTSR2), expressed by neurons and astrocytes, respectively. Downstream of NTSR1 the neuropeptide activated an inward current, calcium release from intracellular stores and, postsynaptically, increased frequency and amplitude of inhibitory synaptic events. NTSR2 activation in astrocytes resulted in increased excitatory input in preoptic GABAergic neurons, an effect which was dependent upon the activation of P2X4 receptors. We also found that neuromedin N acted as a selective agonist at the NTSR1. Surprisingly, activation of both NTSR1 and NTSR2 in the median preoptic nucleus was required for activating a full hypothermic response.

Diverse actions of the modulatory peptide neurotensin on central synaptic transmission

Neurotensin (NT) is a 13 amino acid neuropeptide that is expressed throughout the central nervous system and is implicated in the etiology of multiple diseases and disorders. Many primary investigations of NT-induced modulation of neuronal excitability at the level of the synapse have been conducted, but they have not been summarized in review form in nearly 30 years. Therefore, the goal of this review is to discuss the many actions of NT on neuronal excitability across brain regions as well as NT circuit architecture. In the basal ganglia as well as other brain nuclei, NT can act through diverse intracellular signaling cascades to enhance or depress neuronal activity by modulating activity of ion channels, ionotropic and metabotropic neurotransmitter receptors, and presynaptic release of neurotransmitters. Further, NT can produce indirect effects by evoking endocannabinoid release, and recently has itself been identified as a putative retrograde messenger. In the basal ganglia, the diverse actions and circuit architecture of NT signaling allow for input-specific control of reward-related behaviors.

The Neurotensin NTS1 Receptor Agonist PD149163 Produces Antidepressant-Like Effects in the Forced Swim Test: Further Support for Neurotensin as a Novel Pharmacologic Strategy for Antidepressant Drugs

Preclinical Research Neurotensin is a nonbrain penetrant neuropeptide neurotransmitter that alters dopaminergic and serotonergic neurotransmission. Previous animal behavioral studies have demonstrated that intra-ventral tegmental administration of neurotensin and system administration of the selective neurotensin NTS₁ receptor agonist, PD149163 produce antidepressant-like effects in a forced swim test and a differential reinforcement of low rate task, respectively. The present study sought to expand upon these past findings by assessing systemic administration of PD149163 in a forced swim test, a primary antidepressant preclinical screening model, in mice. The tricyclic antidepressant drug imipramine was tested for comparison, and both compounds were also assessed in an open field test. Both PD149163 and imipramine reduced time spent immobile, an antidepressant-like effect, in the forced swim test. The highest dose of each compound significantly reduced locomotor activity. These findings provide further evidence for the putative antidepressant effects for PD149163 and suggest that NTS₁ receptor activation may be a novel pharmacologic strategy for antidepressant drug development. Drug Dev Res 78 : 196-202, 2017. © 2017 Wiley Periodicals, Inc.

Desynchronization of the Rat Cortical Network and Excitation of White Matter Neurons by Neurotensin

Cortical network activity correlates with vigilance state: Deep sleep is characterized by slow, synchronized oscillations, whereas desynchronized, stochastic discharge is typical of the waking state. Neuropeptides, such as orexin and substance P but also neurotensin (NT), promote arousal. Relatively little is known about if NT can directly affect the cortical network, and if so, through which mechanisms and cellular targets. Here, we addressed these issues using rat in vitro cortex preparations. Following NT application specifically to deeper layers, slow oscillation activity was attenuated with a significant reduction in UP state frequency. The cortical response to thalamic stimulation exhibited enhanced temporal precision in the presence of NT, consistent with the transition in vivo from sleep to wakefulness. These changes were associated with a relative shift toward inhibition in the excitation/inhibition balance. Whole-cell recordings from layer 6 revealed presynaptically driven NT-induced inhibition of pyramidal neurons and excitation of fast-spiking interneurons. Deeper in the cortex, neurons within the white matter (WM) were strongly depolarized by NT application. The colocalization of NT and tyrosine hydroxylase immunoreactivities in deep layer fibers throughout the cortical mantle indicates mediation via dopaminergic systems. These data suggest a cortical mechanism for NT-induced wakefulness and support a role for WM neurons in state control.

Neurotensin NTS1 and NTS2 receptor agonists produce anxiolytic-like effects in the 22-kHz ultrasonic vocalization model in rats

Neurotensin is a neuropeptide neurotransmitter that interacts with multiple neurotransmitter systems, including those regulating amygdalar function, via NTS₁ and NTS₂ receptors. Both receptors are expressed in the amygdala and agonists for NTS₁ or NTS₂ receptors have exhibited anxiolytic effects in animal models. Systemic adminstration of NTS₁ receptor agonist PD149163 was recently shown to reduce footshock conditioned 22-kHz ultrasonic vocalizations in rats, suggesting that PD149163 produced an anxiolytic effect. The effects that neurotensin may have or a selective NTS₂ receptor agonist may have on 22-kHz vocalizations has yet to be examined. The current study evaluated the effects of intracerebroventricularly administered neurotensin (0.1-10.0μg), PD149163 (0.1-10.0ng), or the NTS₂ receptor agonist JMV-431 (0.1-1.0μg) on footshock conditioned 22-kHz vocalizations in male Wistar rats. Neurotensin, PD149163, and JMV-431 all significantly reduced the number 22-kHz calls. No changes in call duration were found, suggesting that non-specific drug effects do not account for the reductions in 22-kHz calls. These data support anxiolytic effects produced by activation of NTS₁ or NTS₂ receptors, and suggest that neurotensin plays a natural role in the expression of conditioned USVs. These data suggest that both receptor subtypes are putative pharmacologic targets.

Diverse roles of neurotensin agonists in the central nervous system

Neurotensin (NT) is a tridecapeptide that is found in the central nervous system (CNS) and the gastrointestinal tract. NT behaves as a neurotransmitter in the brain and as a hormone in the gut. Additionally, NT acts as a neuromodulator to several neurotransmitter systems including dopaminergic, sertonergic, GABAergic, glutamatergic, and cholinergic systems. Due to its association with such a wide variety of neurotransmitters, NT has been implicated in the pathophysiology of several CNS disorders such as schizophrenia, drug abuse, Parkinson's disease (PD), pain, central control of blood pressure, eating disorders, as well as, cancer and inflammation. The present review will focus on the role that NT and its analogs play in schizophrenia, endocrine function, pain, psychostimulant abuse, and PD.

Neurotensin inhibits background K+ channels and facilitates glutamatergic transmission in rat spinal cord dorsal horn

Neurotensin (NT) is a neuropeptide involved in the modulation of nociception. We have investigated the actions of NT on cultured postnatal rat spinal cord dorsal horn (DH) neurons. NT induced an inward current associated with a decrease in membrane conductance in 46% of the neurons and increased the frequency of glutamatergic miniature excitatory synaptic currents in 37% of the neurons. Similar effects were observed in acute slices. Both effects of NT were reproduced by the selective NTS1 agonist JMV449 and blocked by the NTS1 antagonist SR48692 and the NTS1/NTS2 antagonist SR142948A. The NTS2 agonist levocabastine had no effect. The actions of NT persisted after inactivation of G(i/o) proteins by pertussis toxin but were absent after inactivation of protein kinase C (PKC) by chelerythrine or inhibition of the MAPK (ERK1/2) pathway by PD98059. Pre- and postsynaptic effects of NT were insensitive to classical voltage- and Ca(2+) -dependent K(+) channel blockers. The K(+) conductance inhibited by NT was blocked by Ba(2+) and displayed no or little inward rectification, despite the presence of strongly rectifying Ba(2+) -sensitive K(+) conductance in these neurons. This suggested that NT blocked two-pore domain (K2P) background K(+) -channels rather than inwardly rectifying K(+) channels. Zn(2+) ions, which inhibit TRESK and TASK-3 K2P channels, decreased NT-induced current. Our results indicate that in DH neurons NT activates NTS1 receptors which, via the PKC-dependent activation of the MAPK (ERK1/2) pathway, depolarize the postsynaptic neuron and increase the synaptic release of glutamate. These actions of NT might modulate the transfer and the integration of somatosensory information in the DH.

Neurotensin inhibition of GABAergic transmission via mGluR-induced endocannabinoid signalling in rat periaqueductal grey

Neurotensin modulates pain via its actions within descending analgesic pathways which include brain regions such as the midbrain periaqueductal grey (PAG). The aim of this study was to examine the cellular actions of neurotensin on PAG neurons. Whole cell patch clamp recordings were made from rat midbrain PAG slices in vitro to examine the postsynaptic effects of neurotensin and its effects on GABA(A) mediated inhibitory postsynaptic currents (IPSCs). Neurotensin (100-300 nM) produced an inward current in subpopulations of opioid sensitive and insensitive PAG neurons which did not reverse over membrane potentials between -50 and -130 mV. The neurotensin induced current was abolished by the NTS1 and NTS1/2 antagonists SR48692 (300 nM) and SR142948A (300 nM). Neurotensin also produced a reduction in the amplitude of evoked IPSCs, but had no effect on the rate and amplitude of TTX-resistant miniature IPSCs. The neurotensin induced inhibition of evoked IPSCs was reduced by the mGluR5 antagonist MPEP (5microM) and abolished by the cannabinoid CB(1) receptor antagonist AM251 (3 microM). These results suggest that neurotensin produces direct neuronal depolarisation via NTS1 receptors and inhibits GABAergic synaptic transmission within the PAG. The inhibition of synaptic transmission is mediated by neuronal excitation and action potential dependent release of glutamate, leading to mGluR5 mediated production of endocannabinoids which activate presynaptic CB(1) receptors. Thus, neurotensin has cellular actions within the PAG which are consistent with both algesic and analgesic activity, some of which are mediated via the endocannabinoid system.

Stimulation by neurotensin of dopamine and 5-hydroxytryptamine (5-HT) release from rat prefrontal cortex: possible role of NTR1 receptors in neuropsychiatric disorders

The modulation of cortical dopaminergic and serotonergic neurotransmissions by neurotensin (NT) was studied by measuring the release of dopamine (DA) and 5-hydroxytryptamine (5-HT) from the prefrontal cortex (PFC) of freely moving rats. The samples were collected via transversal microdialysis. Dopamine and 5-HT levels in the dialysate were measured using high-performance liquid chromatography (HPLC) with an electrochemical detector. Local administration of neurotensin (1microM or 0.1microM) in the PFC via the dialysis probe produced significant, long-lasting, and concentration-dependent increase in the extracellular release of DA and 5-HT. The increase produced by 1microM neurotensin reached a maximum of about 210% for DA and 340% for 5-HT. A high-affinity selective neurotensin receptor (NTR1) antagonist {2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxyphenyl)pyrazol-3yl)carbonylamino tricyclo (3.3.1.1.(3.7)) decan-2-carboxylic acid} (SR 48692), perfused locally at a concentration of 0.1microM and 0.5microM in the PFC antagonized the effects of 1microM neurotensin. Our in vivo neurochemical results indicate, for the first time, that neurotensin is able to regulate cortical dopaminergic and serotonergic neuronal activity in freely moving rats. These effects are possibly mediated by interactions of neurotensin with neurons releasing DA or 5-HT, projecting to the PFC from the ventrotegmental area (VTA) and from the dorsal raphe nuclei (DRN), respectively. The potentiating effects of neurotensin on DA and 5-HT release in the PFC are regulated by NTR1 receptors, probably located on dopaminergic and serotonergic nerve terminals or axons.

Neurotensin effects on N-type calcium currents among rat pallidal neurons: an electrophysiological and immunohistochemical study

The tridecapeptide neurotensin (NT) is involved in the modulation of dopamine (DA)-mediated functions in the nigrostriatal and mesocorticolimbic pathways. Its relevance in mammalian globus pallidus (GP) is questioned. A recent electrophysiological study on GP slices described NT-mediated robust membrane depolarization, depending upon the suppression of potassium conductance and/or the activation of cation current. Here, we have studied whether NT also affected high-voltage-activated calcium (Ca(2+)) currents, by means of whole-cell recordings on isolated GP neurons. In our hands, the full peptide and the segment NT8-13 reversibly inhibited N-like Ca(2+) current in about 60% of the recorded dissociated neurons, irrespective of their capacitance. The NT-mediated modulation showed no desensitization and was antagonized by the NT1 antagonists SR48692 and SR142948. These results imply an abundant expression of NTS(1) on GP cell somata. Then, we performed a light and immunofluorescence-confocal microscopy study of NTS(1) localization among GP neurons. We found that NTS(1) is localized in about 56% of GP neurons in both subpopulations of neurons, namely parvalbumin positive and negative. We conclude that NT, likely released from the striatal terminals in GP, acts through the postsynaptic NTS(1) preferentially localized in the lateral aspects of the GP. These data suggest a new implication (neither merely presynaptic nor simply "excitatory") for NT in the modulation of GP firing pattern. In addition, NT might have a role in affecting the interplay among the endogenous release of GABA/glutamate and DA. This hypothesis might have implications on both sensori-motor and associative functions of the GP and should be tested in DA-denervated disease models.

Neurotensin and pain modulation

Neurotensin (NT) can produce a profound analgesia or enhance pain responses, depending on the circumstances. Recent evidence suggests that this may be due to a dose-dependent recruitment of distinct populations of pain modulatory neurons. NT knockout mice display defects in both basal nociceptive responses and stress-induced analgesia. Stress-induced antinociception is absent in these mice and instead stress induces a hyperalgesic response, suggesting that NT plays a key role in the stress-induced suppression of pain. Cold water swim stress results in increased NT mRNA expression in hypothalamic regions known to project to periaqueductal gray, a key region involved in pain modulation. Thus, stress-induced increases in NT signaling in pain modulatory regions may be responsible for the transition from pain facilitation to analgesia. This review focuses on recent advances that have provided insights into the role of NT in pain modulation.

Neurotensin modulates synaptic transmission in the nucleus of the solitary tract of the rat

Whole-cell patch clamp recordings were made from neurons of the rat subpostremal region of the nucleus tractus solitarius (NTS) in transverse brainstem slices. Neurotensin (NT) enhanced the firing rate of action potentials from 0.8 +/- 0.4 Hz in control to 1.9 +/- 1.3 Hz (n = 9) and increased their decay time. The peak amplitude of the after-hyperpolarization was decreased by 34+/-5% (n = 9). These effects were associated with a depolarization of 4 +/- 1 mV (n = 10) in the resting membrane potential and an increase in the input resistance (from 768 +/- 220 MOmega to 986+/-220 MOmega; n = 5) and were compensated by manually hyperpolarizing the cell to control values. In voltage clamp experiments NT decreased an outward current (from 488 +/- 161 to 340 +/- 96 pA at +40 mV; n = 5) which reversed near the potassium equilibrium potential. In addition, NT increased the frequency of both excitatory and inhibitory spontaneous synaptic currents, an effect blocked by tetrodotoxin, and did not change the evoked excitatory or inhibitory postsynaptic currents. The selective NTR1 receptor antagonist SR48692 reversibly blocked the effects of NT on both action potentials and spontaneous synaptic currents. Our results suggest that NTR1 receptors can modulate post-synaptic responses in neurons of the subpostremal NTS by increasing cell excitability as a result of blockade of a potassium conductance.

Neurotensin agonists block the prepulse inhibition deficits produced by a 5-HT2A and an alpha1 agonist

RATIONALE

Neurotensin (NT) agonists have been proposed as potential antipsychotics based exclusively upon their ability to inhibit dopamine-2 (D2) receptor transmission. Several other pharmacological mechanisms have been implicated in enhancing the antipsychotic profile produced by D2 inhibition alone. These include inhibition of 5-HT2A and alpha1-adrenoceptors. Recently, we reported that systemic administration of the neurotensin agonist PD149163 blocks deficits in prepulse inhibition (PPI) of the startle reflex produced by the 5-HT2A receptor agonist DOI. This suggested that NT agonists could inhibit 5-HT2A modulation of neurotransmission.

OBJECTIVE

To determine if other peripherally administered NT agonists shared this effect, we examined the effects of NT69L, another NT agonist, on DOI-induced PPI deficits. In addition, to determine if NT agonists also inhibit alpha1-adrenoceptor neurotransmission, we examined the effects of PD149163 and NT69L on PPI deficits induced by the alpha1-adrenoceptor agonist, cirazoline.

METHODS

In the NT69L/DOI study, rats received subcutaneous (SC) injections of NT69L (0, 0.1, 1, or 2 mg/kg) followed 30 min later by SC saline or DOI (0.5 mg/kg). In the NT agonist/cirazoline studies, animals received SC injections of either PD149163 (0, 0.01, 0.1, or 1 mg/kg) or NT69L (0, 0.01, 0.1, or 1 mg/kg) followed 30 min later by SC saline or cirazoline (0.7 mg/kg). Animals were tested in startle chambers 20 min later.

RESULTS

In all three experiments the PPI disruption produced by DOI and cirazoline was blocked by the NT agonists.

CONCLUSIONS

These findings provide strong evidence that NT agonists inhibit 5-HT2A and alpha1-adrenoceptor modulation of neurotransmission, pharmacological effects that, in conjunction with their known inhibition of dopamine transmission, strengthen the antipsychotic potential of NT agonists.

Neurotensin stimulates both calcium mobilization from inositol trisphosphate-sensitive intracellular stores and calcium influx through membrane channels in frog pituitary melanotrophs

Neurotensin (NT) is a potent stimulator of electrical and secretory activities in frog pituitary melanotrophs. The aim of the present study was to characterize the transduction pathways associated with activation of NT receptors in frog melanotrophs. Application of synthetic frog NT (fNT) increased the cytosolic calcium concentration ([Ca2+]c) and stimulated the formation of inositol trisphosphate (IP3). The phospholipase C inhibitor U-73122 blocked the electrophysiological and secretory effects of fNT. Intracellular application of the IP3 receptor antagonist heparin abolished fNT-induced electrical activity. Suppression of Ca2+ in the incubation medium markedly reduced the effect of NT on [Ca2+]c, firing rate, and alpha-melanocyte-stimulating hormone (alphaMSH) secretion. Similarly, the inhibitor of IP3-induced Ca2+ release and store-operated Ca2+ channels, 2-Aminoethoxydiphenylborane, and the nonselective Ca2+ channel blockers GdCl3 and NiCl2, attenuated the [Ca2+]c increase and the electrical and secretory responses evoked by fNT. Coapplication of the L- and N-type Ca2+ channel blockers nifedipine and omega-CgTx GVIA reduced the effects of fNT on action potential discharge, [Ca2+]c increase, and alphaMSH release. The protein kinase C (PKC) inhibitors, PKC-(19-31) and chelerythrine, reduced the electrophysiological and secretory responses induced by iterative applications of fNT. Collectively, these results demonstrate that, in frog melanotrophs, NT stimulates the phospholipase C/PKC pathway and increases [Ca2+]c. Both Ca2+ release from intracellular stores and Ca2+ influx through L- and N-type Ca2+ channels are involved in fNT-induced alphaMSH secretion. In addition, the present data indicate that PKC plays a crucial role in maintenance of the responsiveness of melanotrophs to NT.

Hypocretins (orexins) regulate serotonin neurons in the dorsal raphe nucleus by excitatory direct and inhibitory indirect actions

The hypocretins (hcrt1 and hcrt2) are expressed by a discrete population of hypothalamic neurons projecting to many regions of the CNS, including the dorsal raphe nucleus (DRN), where serotonin (5-HT) neurons are concentrated. In this study, we investigated responses to hcrts in 216 physiologically identified 5-HT and non-5-HT neurons of the DRN using intracellular and whole-cell recording in rat brain slices. Hcrt1 and hcrt2 induced similar amplitude and dose-dependent inward currents in most 5-HT neurons tested (EC50, approximately 250 nm). This inward current was not blocked by the fast Na+ channel blocker TTX or in a Ca2+-free solution, indicating a direct postsynaptic action. The hcrt-induced inward current reversed near -18 mV and was primarily dependent on external Na+ but not on external or internal Ca2+, features typical of Na+/K+ nonselective cation channels. At higher concentrations, hcrts also increased spontaneous postsynaptic currents in 5-HT neurons (EC50, approximately 450-600 nm), which were TTX-sensitive and mostly blocked by the GABA(A) antagonist bicuculline, indicating increased impulse flow in local GABA interneurons. Accordingly, hcrts were found to increase the basal firing of presumptive GABA interneurons. Immunolabeling showed that hcrt fibers projected to both 5-HT and GABA neurons in the DRN. We conclude that hcrts act directly to excite 5-HT neurons primarily via a TTX-insensitive, Na+/K+ nonselective cation current, and indirectly to activate local inhibitory GABA inputs to 5-HT cells. The greater potency of hcrts in direct excitation compared with indirect inhibition suggests a negative feedback function for the latter at higher levels of hcrt activity.

Histamine H1 receptors in rat dorsal raphe nucleus: pharmacological characterisation and linking to increased neuronal activity

In this work we studied the presence of histamine H(1) receptors in the rat dorsal raphe nucleus (DRN) and the effect of their activation on the activity of presumed serotonergic DRN neurones. [(3)H]-Mepyramine bound to DRN membranes with best-fit values of 107+/-13 fmol/mg protein for maximum binding (B(max)) and 1.2+/-0.4 nM for the equilibrium dissociation constant (K(d)). In DRN slices labelled with [(3)H]-inositol and in the presence of 10 mM LiCl, histamine stimulated the accumulation of [(3)H]-inositol phosphates ([(3)H]-IPs) with maximum effect 172+/-6% of basal and EC(50) 3.2+/-1.3 microM. [(3)H]-IPs accumulation induced by 100 microM histamine (162+/-5% of basal) was markedly, but not fully blocked by the selective H(1) antagonist mepyramine (300 nM; 64+/-6% inhibition). The simultaneous addition of mepyramine and the selective H(2) antagonist ranitidine (10 microM) abolished histamine-induced [(3)H]-IPs accumulation. The presence of H(2) receptors was confirmed by [(3)H]-tiotidine binding and by the determination of histamine-induced [(3)H]-cyclic AMP formation. Extracellular single-unit recording in brain stem slices showed that the exposure to histamine resulted in a marked increase in the firing rate of DRN presumed serotonergic neurones (471+/-10% of basal), that was dependent on the concentration of the agonist (EC(50) 4.5+/-0.3 microM). The action of histamine was not affected by the H(2) antagonist tiotidine (2 microM) but was fully prevented by 1 microM mepyramine. Taken together, our results indicate that histamine modulates the firing of DRN presumed serotonergic neurones through the activation of H(1) receptors coupled to phosphonositide hydrolysis.

Presynaptic action of neurotensin on cultured ventral tegmental area dopaminergic neurones

Dopamine-containing neurones of the ventral tegmental area express neurotensin receptors which are involved in regulating cell firing and dopamine release. Although indirect evidence suggests that some neurotensin receptors may be localised on the nerve terminals of dopaminergic neurones in the striatum and thus locally regulate dopamine release, a clear demonstration of such a mechanism is lacking and a number of indirect sites of action are possible. We have taken advantage of a simplified preparation in which cultured rat ventral tegmental area dopaminergic neurones establish nerve terminals that co-release glutamate to determine whether neurotensin can act at presynaptic sites. We recorded glutamate-mediated synaptic currents that were generated by dopaminergic nerve terminals as an index of presynaptic function. The neurotensin receptor agonist NT(8-13) caused an inward current and an enhancement of the firing rate of dopaminergic neurones together with an increase in the frequency of spontaneous glutamate receptor-mediated excitatory postsynaptic currents (EPSCs). Incompatible with a direct excitatory action on nerve terminals, NT(8-13) failed to change the amplitude of individual action potential-evoked EPSCs or the frequency of miniature EPSCs recorded in the presence of tetrodotoxin. However, NT(8-13) reduced the ability of terminal D2 dopamine receptors to inhibit action potential-evoked EPSCs in isolated dopaminergic neurones. Taken together, our results suggest that in addition to its well-known somatodendritic excitatory effect leading to an increase in firing rate, neurotensin also acts on nerve terminals. The main effect of neurotensin on nerve terminals is not to produce a direct excitation, but rather to decrease the effectiveness of D2 receptor-mediated presynaptic inhibition.

Hypocretin (orexin) enhances neuron activity and cell synchrony in developing mouse GFP-expressing locus coeruleus

The noradrenergic neurons of the locus coeruleus (LC) play an important role in modulating arousal and selective attention. A similar function has been attributed to the hypocretin neurons of the hypothalamus which maintain a strong synaptic projection to the LC. As the LC can be difficult to detect in the embryonic and neonatal mouse brain, we used a new transgenic mouse with strong GFP expression in the LC under the regulation of a mouse prion promoter. GFP colocalized with immunoreactive tyrosine hydroxylase in sections and dispersed cultures of the LC, allowing visualization and whole cell or single-unit recording from the LC in early stages of cellular development. GFP expression in the LC had no apparent effect on cellular physiology, including resting membrane potential, input resistance, spike threshold, depolarization-induced spike frequency increase, current-voltage relations, or hypocretin responses. In slices of the mature mouse and rat LC, hypocretin-1 and -2 increased spike frequency, with hypocretin-1 being an order of magnitude more potent. In the postnatal day (P) 0-2 developing mouse slice during a developmental period when spikes could be elicited in some cells, other developing LC neurons showed rhythmic, subthreshold oscillations (approximately 1 Hz) in membrane potential (2.9-7.4 mV amplitude); others were arrhythmic. Hypocretin-1 depolarized the membrane potential, resulting in the appearance of spikes in developing LC cells that showed no spikes under control conditions. In the presence of TTX and glutamate receptor antagonists, hypocretin-1-mediated inward currents were blocked by substitution of choline-Cl for NaCl, suggesting an excitatory mechanism based on an inward cation current. Hypocretin-1 initiated strong regular membrane voltage oscillations in arrhythmic immature neurons. Hypocretin increased the temporal synchrony of action potentials studied with dual-cell recording in P1-P5 mouse LC slices, consistent with the view that synchrony of LC output, associated with improved cognitive performance, may be increased by hypocretin. Together these data suggest that the hypothalamus, via hypocretin projections, may therefore be in a position to enhance arousal and modulate plasticity in higher brain centres through the developing LC.

Neurotensin excitation of serotonergic neurons in the rat nucleus raphe magnus: ionic and molecular mechanisms

To understand the cellular and molecular mechanisms by which neurotensin (NT) induces an analgesic effect in the nucleus raphe magnus (NRM), whole-cell patch-clamp recordings were performed to investigate the electrophysiological effects of NT on acutely dissociated NRM neurons. Two subtypes of neurons, primary serotonergic and secondary non-serotonergic cells, were identified from acutely isolated NRM neurons. During current-clamp recordings, NT depolarized NRM serotonergic neurons and evoked action potentials. Voltage-clamp recordings showed that NT excited serotonergic neurons by enhancing a voltage-insensitive and non-selective cationic conductance. Both SR48692, a selective antagonist of subtype 1 neurotensin receptor (NTR-1), and SR 142948A, a non-selective antagonist of NTR-1 and subtype 2 neurotensin receptor (NTR-2), failed to prevent neurotensin from exciting NRM serotonergic neurons. NT-evoked cationic current was inhibited by the intracellular administration of GDP-beta-S. NT failed to induce cationic currents after dialyzing serotonergic neurons with the anti-G(alphaq/11) antibody. Cellular Ca(2+) imaging study using fura-2 showed that NT induced the calcium release from the intracellular store. NT-evoked current was blocked after the internal perfusion of heparin, an IP(3) receptor antagonist, or BAPTA, a fast Ca(2+) chelator. It is concluded that neurotensin enhancement of the cationic conductance of NRM serotonergic neurons is mediated by a novel subtype of neurotensin receptors. The coupling mechanism via G(alphaq/11) proteins is likely to involve the generation of IP(3), and subsequent IP(3)-evoked Ca(2+) release from intracellular stores results in activating the non-selective cationic conductance.

Neurotensin excites periaqueductal gray neurons projecting to the rostral ventromedial medulla

Microinjection of neurotensin into the midbrain periaqueductal gray (PAG) produces a potent and naloxone-insensitive analgesic effect. To test the hypothesis that neurotensin induces the analgesic effect by activating the PAG-rostral ventromedial medulla (RVM) descending antinociceptive pathway, PAG neurons that project to RVM (PAG-RVM) were identified by microinjecting DiI(C18), a retrograde tracing dye, into the rat RVM. Subsequently, fluorescently labeled PAG-RVM projection neurons were acutely dissociated and selected for whole cell patch-clamp recordings. During current-clamp recordings, neurotensin depolarized retrogradely labeled PAG-RVM neurons and evoked action potentials. Voltage-clamp recordings indicated that neurotensin excited PAG-RVM neurons by opening the voltage-insensitive and nonselective cation channels. Both SR 48692, a selective NTR-1 antagonist, and SR 142948A, a nonselective antagonist of NTR-1 and NTR-2, failed to prevent neurotensin from exciting PAG-RVM neurons. Neurotensin failed to evoke cationic currents after internally perfusing PAG-RVM projection neurons with GDP-beta-S or anti-G(alpha q/11) antiserum. Cellular Ca(2+) fluorescence measurement using fura-2 indicated that neurotensin rapidly induced Ca(2+) release from intracellular stores of PAG-RVM neurons. Neurotensin-evoked cationic currents were blocked by heparin, an IP(3) receptor antagonist, and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a fast chelator of Ca(2+). These results suggest that by activating a novel subtype of neurotensin receptors, neurotensin depolarizes and excites PAG-RVM projection neurons through enhancing Ca(2+)-dependent nonselective cationic conductance. The coupling mechanism via G(alpha q/11) proteins is likely to involve the production of IP(3), and subsequent IP(3)-evoked Ca(2+) release leads to the opening of nonselective cation channels.

Serotonin 5-HT(2) receptors activate local GABA inhibitory inputs to serotonergic neurons of the dorsal raphe nucleus

The purpose of the present study was to characterize the synaptic currents induced by bath-applied serotonin (5-HT) in 5-HT cells of the dorsal raphe nucleus (DRN) and to determine which 5-HT receptor subtypes mediate these effects. In rat brain slices, 5-HT induced a concentration-dependent increase in the frequency of inhibitory postsynaptic currents (IPSCs) in 5-HT neurons recorded intracellularly in the ventral part of the DRN (EC(50): 86 microM); 5-HT also increased IPSC amplitude. These effects were blocked by the GABA(A) receptor antagonist, bicuculline (10 microM) and by the fast sodium channel blocker, TTX, suggesting that 5-HT had increased impulse flow in local GABAergic neurons. DAMGO (300 nM), a selective mu-agonist, markedly suppressed the increase in IPSC frequency induced by 5-HT (100 microM) in the DRN. A near maximal concentration of the selective 5-HT(2A) antagonist, MDL100,907 (30 nM), produced a large reduction ( approximately 70%) in the increase in IPSC frequency induced by 100 microM 5-HT; SB242,084 (30 nM), a selective 5-HT(2C) antagonist, was less effective ( approximately 24% reduction). Combined drug application suppressed the increase in 5-HT-induced IPSC frequency almost completely, suggesting involvement of both 5-HT(2A) and 5-HT(2C) receptors. Unexpectedly, the phenethylamine hallucinogen, DOI, a partial agonist at 5-HT(2A/2C) receptors, caused a greater increase (+334%) in IPSC frequency than did 5-HT 100 microM (+80%). This result may be explained by an opposing 5-HT(1A) inhibitory effect since the selective 5-HT(1A) antagonist, WAY-100635, enhanced the 5-HT-induced increase in IPSCs. These results indicate that within the DRN-PAG area there may be a negative feedback loop in which 5-HT induces an increase in IPSC frequency in 5-HT cells by exciting GABAergic interneurons in the DRN via 5-HT(2A) and, to a lesser extent, 5-HT(2C) receptors. Increased GABA tone may explain the previous observation of an indirect suppression of firing of a subpopulation of 5-HT cells in the DRN induced by phenethylamine hallucinogens in vivo.

Distribution of the neurotensin receptor NTS1 in the rat CNS studied using an amino-terminal directed antibody

The distribution of neurotensin receptor 1 immunoreactivity in the rat brain was studied using an antibody against the amino-terminal of the receptor expressed as a fusion protein with glutathione-S transferase. Affinity purified antibodies detected the fusion protein and the complete neurotensin receptor sequence expressed in Escherichia coli. The immunostaining was abolished by preabsorption with the amino-terminal fusion protein. Immunoreactive neurotensin receptor 1 immunoreactivity was detected on cell bodies and their processes in a number of CNS regions. In agreement with previous binding studies neurotensin receptor 1 immunoreactivity was particularly localised in cell bodies in the basal forebrain, nucleus basalis and substantia nigra. At the electron microscope level immunoreactivity was found both in axonal bouton and dendrites and spines in the basal forebrain indicating that neurotensin may act both pre- and post-synaptically. There were several regions such as the substantia gelatinosa, ventral caudate-putamen and the lateral reticular nucleus where the neurotensin receptor 1 positive cells had not previously been reported, indicating that distribution of this receptor is widespread.

Neurotensin increases endogenous glutamate release in rat cortical slices

In the present study, the effects of the tridecapeptide neurotensin [NT(1-13)] and its fragments, NT(1-7) and NT(8-13), on endogenous glutamate release from rat cortical slices, were evaluated. NT(1-13) (100-1000 nM) slightly increased spontaneous glutamate release, while it was ineffective at 1 and 10 nM concentrations. Neither the biologically active NT fragment NT(8-13) nor the inactive one NT(1-7) affected basal glutamate release. NT(1-13) (1-1000 nM) enhanced potassium (35 mM)-evoked glutamate release displaying a bell-shaped concentration response curve. In addition NT(8-13) (10 nM) increased K+-evoked-glutamate release similarly to the parent peptide (10 nM), while the biologically inactive fragment NT(1-7) (10-100 nM) was ineffective. The effects of NT(1-13) and NT(8-13) were fully counteracted by the selective neurotensin receptor antagonist SR48692 (100 nM). These findings suggest that NT plays a role in regulating cortical glutamate transmission.

Chronic morphine increases GABA tone on serotonergic neurons of the dorsal raphe nucleus: association with an up-regulation of the cyclic AMP pathway

Major adaptations after chronic exposure to morphine include an up-regulation of the adenosine 3',5'-monophosphate pathway. Acute opioids, via mu-opioid receptors, disinhibit midbrain serotonergic neurons by suppressing inhibitory GABAergic transmission in the dorsal raphe nucleus and adjacent periaqueductal gray. This study examined whether chronic morphine induces a compensatory increase in GABA inputs to 5-hydroxytryptamine neurons and whether this was associated with an up-regulation of the adenosine 3',5'-monophosphate pathway. The firing rate of serotonergic neurons was reduced in brain slices from morphine-dependent rats, an effect reversed by the GABA(A) antagonist bicuculline. The reduction in firing rate was accompanied by an increased frequency of spontaneous GABAergic inhibitory postsynaptic currents, indicating increased GABA tone in the slice. The increase in GABA tone in brain slices from dependent rats was associated with increased induction of inhibitory postsynaptic currents by the adenylyl cyclase activator forskolin, suggesting an up-regulation of the adenosine 3',5'-monophosphate pathway. Indeed, chronic morphine increased levels of adenylyl cyclase VIII (but not of adenylyl cyclase I, III or V) immunoreactivity in the dorsal raphe nucleus area. Two adenosine 3',5'-monophosphate-mediated mechanisms for the increase in GABA tone were discerned. The first, which predominated when impulse-flow was blocked by tetrodotoxin, involves protein kinase A since it was sensitive to protein kinase A inhibitors. The second, seen when impulse-flow was intact (i.e. absence of tetrodotoxin), was insensitive to protein kinase A inhibitors but was suppressed by ZD7288, a blocker of hyperpolarizing-activated Ih channels which are directly activated by adenosine 3',5'-monophosphate. We conclude that chronic morphine induces an up-regulation of the adenosine 3',5'-monophosphate pathway in GABAergic inputs to serotonergic cells, resulting in an increase in spontaneous and impulse-flow dependent GABA release. These changes would lead to an increase in GABA tone and subsequently to the reported decrease in serotonergic activity during opiate withdrawal.

Neurotensin depolarizes cholinergic and a subset of non-cholinergic septal/diagonal band neurons by stimulating neurotensin-1 receptors

Identified cholinergic and a subtype of non-cholinergic, fast-firing neurons were recorded intracellularly in vitro from slices of guinea-pig brain. Recorded neurons were within the boundaries of the medial septum and vertical limb of the diagonal band of the forebrain. The effects of superfused neurotensin and neurotensin receptor antagonists were measured under single-electrode current clamp. Neurotensin consistently caused a dose-dependent, slow depolarization of cholinergic neurons that was accompanied by an increase in membrane resistance and a block of the long-duration (1-10 s) post-spike afterhyperpolarization when present. Neurotensin also blocked a shorter duration, slow afterhyperpolarization, but only in a minority of cholinergic neurons. When present, inhibition of the slow afterhyperpolarization changed the spike pattern from single spikes to short bursts. Inhibition of post-spike afterhyperpolarizations by neurotensin reversed more slowly than did other effects of neurotensin. Tetrodotoxin did not prevent the depolarizing effect of neurotensin. The non-selective neurotensin receptor antagonist, SR142948A, blocked the depolarizing effect of neurotensin but the low-affinity receptor antagonist, levocabastine, did not. A subgroup of noncholinergic, fast-firing neurons (23%) was also depolarized by neurotensin, an effect antagonized by SR142948A but not levocabastine. Neurotensin did not effect post-spike voltage transients or change the firing pattern of non-cholinergic neurons. These data suggest that neurotensin causes a slow depolarization and increased excitability of cholinergic and some noncholinergic neurons in an area of the brain that projects to the hippocampus. Neurotensin type 1 receptors appear to mediate these effects. Neurotensin may modulate hippocampal-dependent learning and memory processes through its effects on septohippocampal neurons.

Role of endogenous neurotensin in the behavioral and neuroendocrine effects of cocaine

The present experiments were designed to assess the role of endogenous neurotensin (NT) in the behavioral response to acute and daily cocaine, after administration of the NT receptor antagonist, SR 48692. Given that glucocorticoids increase the sensitivity to the psychomotor effects of drugs of abuse, we also investigated the effects of SR 48692 on basal and cocaine-induced corticosterone secretion. Acute administration of SR 48692 (1 mg/kg i.p.) reduced the number of rearings induced by cocaine (15 mg/kg i.p.), without modifying horizontal activity. Repeated pretreatment with SR 48692 (1 mg/kg x 5 days) markedly reduced locomotion and rearings after an acute cocaine challenge (day 1), whereas the lower dose of SR 48692 (0.1 mg/kg) had no effect. SR 48692 (1 mg/kg), given daily before cocaine, also decreased cocaine-induced rearing on day 2, but had no effect on the following drug challenges (days 3-10). One week after discontinuing repeated cocaine injections, SR 48692 blocked vertical, but not horizontal, activity induced by an acute cocaine challenge. Rats treated repeatedly with cocaine showed an enhanced behavioral response characterized by the development of stereotypes, which were unaffected by SR 48692. Finally, treatment with SR 48692 did not alter corticosterone circadian secretion nor cocaine-stimulated corticosterone levels, indicating that the attenuation of the behavioral effects of cocaine after NT receptor blockade is not associated with blunted glucocorticoid secretion. These results indicate that administration of SR 48692 attenuates the locomotion and rearing response to cocaine but fails to modify stereotyped behavior, suggesting that SR 48692 modulates the behavioral effects of psychostimulant drugs by acting selectively on the mesolimbic dopaminergic system.

Stimulation by neurotensin of (3H)5-hydroxytryptamine (5HT) release from rat frontal cortex slices

The effect of neurotensin (NT) on the K+-evoked (3H)5HT release from brain frontal cortex slices was studied in rats. NT(1-13) and NT(8-13) increased (3H)5HT release with EC50 values in the nanomolar range and Emax values in the range of 100% of control, whereas D-tyr11-NT was inactive. Concerning NT receptor antagonists, SR 48692 and SR 142948A antagonized with IC50 values of 4.8+/-1.8 nM and 4.5+/-1.8 nM respectively, the NT stimulated K+-evoked (3H)5HT release. SR 48527 also antagonized NT induced (3H)5HT release with an IC50 value of 0.95+/-0.06 nM whereas the inactive R(-) enantiomer SR 49711 only inhibited this effect with IC50 value close to 10(-6)M. The 5HT-releasing effect of NT was completely inhibited by tetrodotoxin suggesting that NT receptors involved in the control of 5-HT release are not located on 5-HT terminals. After a first NT (10(-7)M) application, the NT (10(-7)M, 10(-6)M) effect under K+ depolarization was drastically decreased, indicating that the NT receptor could be desensitized. No potentiating effect of NT on K+-evoked (3H)5HT release was observed in striatal and hippocampal slices. These results suggest that, in the rat frontal cortex, NT regulates 5HT release through a high affinity NT receptor not associated with 5HT terminals.

Electrophysiological evidence for putative subtypes of neurotensin receptors in guinea-pig mesencephalic dopaminergic neurons

Electrophysiologically identified mesencephalic dopaminergic neurons were examined by means of extra- and intracellular microelectrodes in coronal slices of guinea-pig brain. Neurotensin and its C-terminal fragment (8-13) were equipotent in the enhancement of spontaneous neuronal firing rate (EC50 values 81.9 and 72.6nM, respectively). The duration of response was significantly longer and more variable for neurotensin compared to neurotensin fragment (8-13) (mean half-time of recovery 423+/-44 and 100+/-14 s, respectively, for peptides applied at 300 nM). The initial fast phase of excitatory responses to neurotensin receptor agonists was associated with membrane depolarization (when assessed in current-clamp mode) or with inward currents (when assessed in voltage-clamp mode), whereas prolonged excitation was associated with a slowly occurring and long-lasting change in the late afterhyperpolarization. Two kinetically distinct components were revealed in responses to neurotensin and neurotensin fragment (8-13) by the use of SR48692 and SR142948, two selective non-peptide neurotensin receptor antagonists. SR142948 (100 nM) potently antagonized responses to both agonists [response was reduced by 661 5% and 74+/-9% for neurotensin and neurotensin fragment (8-13), respectively] and caused a rightward shift in the concentration-response curve for neurotensin. On the other hand, SR48692 (100 nM) selectively inhibited the slow (late afterhyperpolarization-dependent) component, without altering the response amplitude; the half-time of recovery was reduced by 71+/-6% and 65+/-5% of control values for responses induced by neurotensin (300 nM) and neurotensin fragment (8-13) (300 nM), respectively. In addition, neurotensin, but not neurotensin fragment (8-13), provoked SR48692-sensitive and long-lasting attenuation of dopamine-induced inhibitory responses. It is suggested that two subtypes of neurotensin receptors are present in dopaminergic neurons, based on the differences in agonist and antagonist sensitivity, kinetic properties and the membrane mechanisms involved.

Involvement of potentially distinct neurotensin receptors in neurotensin-induced stimulation of striatal [3H]dopamine release evoked by KCl versus electrical depolarization

We intended to determine whether the effect of neurotensin (NT) on K+ and electrically evoked [3H]dopamine (DA) release from rat and guinea-pig striatal slices involved different mechanisms and/or receptors. In the two species, NT and three NT agonists were found to exhibit different relative potencies to enhance K+- and electrically-evoked [3H]DA release. NT(1-13) increased [3H]DA release with EC50 values in the nanomolar range and Emax values in the range of 100% of control. NT(8-13) and Eisai hexapeptide were both as active as NT(1-13) under K+ depolarization, but did not exceed 40% of the NT(1-13) effect under electrical depolarization. In rats, when [3H]DA release was stimulated with two successive K+ depolarizations, in the presence of NT(1-13), the NT effect during the second exposure to K+ was drastically decreased, suggesting that the NT receptor was desensitized. The desensitization process was essentially observed on Emax values, EC50 values being weakly affected. Similar results were obtained in guinea pig. In contrast, with two electrical depolarizations or with two different depolarizations (K+ followed by electrical), the NT effect during the second depolarization was not significantly affected. Concerning NT antagonists, SR 48692 antagonized with IC50 values in the nanomolar range the NT(1-13) stimulated K+-evoked [3H]DA release but did not affect, up to 10(-6) M, the NT(1-13) enhancement of electrically stimulated [3H]DA release. On the contrary, SR 142948A antagonized the NT(1-13) effect on K+- and electrically-evoked [3H]DA release. In conclusion, these results suggest the possible existence of potentially distinct neurotensin receptors differentially involved in the control exerted by NT on DA release under KCl vs electrical depolarization.

Neurotensin and the serotonergic system

The serotonergic system, because of very diffuse projections throughout the central nervous system, has been implicated in numerous functions including nociception, analgesia, sleep-wakefulness and autonomic regulation. Despite an abundant literature indicating the presence of neurotensin-containing (neurotensinergic) neurons, fibres and terminals in most areas containing serotonergic neurons, little is known about the possible relationship between serotonergic and neurotensinergic systems. The purpose of this review is (i) to summarize current knowledge on the anatomical relation between neurotensinergic and serotonergic system, (ii) to summarize current knowledge on the action of neurotensin on serotonergic neurons and (iii) to discuss the possible physiological relevance of this action. Neurotensin-containing cell bodies can be found in the most rostral raphe nuclei. There are neurotensin-containing fibres and terminals in all raphe nuclei. Raphe nuclei have also been shown to contain neurotensin-receptor binding sites. In the dorsal raphe nucleus, neurotensin induces a concentration-dependent increase in the firing rate of a subpopulation of serotonergic neurons. The neurotensin-induced excitation, which is selectively blocked by the non-peptide neurotensin receptor antagonist SR 48692, is observed mainly in the ventral part of the nucleus. Most serotonergic neurons show marked desensitization to neurotensin, even at low concentrations. In intracellular experiments, neurotensin induces an inward current, associated in some cases with a decrease in apparent input conductance, which is occluded by supramaximal concentrations of the alpha 1-adrenoceptor agonist phenylephrine. In rare cases, neurotensin induces an excitation of GABAergic or glutamatergic neurons. Since the neurotensinergic system has also been implicated in nociception, analgesia, sleep-wakefulness, and autonomic regulation, the review discusses the possibility that part of this regulation could involve the activation of the serotonergic system.

Involvement of cortical neurotensin in the regulation of rat meso-cortico-limbic dopamine neurons: evidence from changes in the number of spontaneously active A10 cells after neurotensin receptor blockade

In order to further assess the role of endogenous neurotensin on midbrain dopaminergic neuronal function, the effects of the selective neurotensin receptor antagonists SR 48692 and SR 48527 were investigated on the number of spontaneously active A9 and A10 dopaminergic neurons in rats. Single intraperitoneal administration of SR 48692 (0.1-3 mg/kg) dose-dependently increased the number of active A10, but not A9 cells. SR 48527 (1 mg/kg) had a similar profile, but not SR 49711, its low affinity R-enantiomer, indicating that the effects observed were mediated through neurotensin receptor blockade. Five-week treatment with SR 48692 (3 mg/kg/day) produced a significant decrease of the number of active A10, but not A9 cells, which was reversed by apomorphine, suggesting that these cells were under depolarization block. Single co-administration of inactive doses of SR 48692 (0.1 mg/kg) and haloperidol (0.0625 mg/kg) significantly increased the number of active A10 cells. Conversely, co-administered active doses of SR 48692 or SR 48527 and haloperidol (1 and 0.25 mg/kg, respectively) induced an apomorphine-sensitive decrease of the number of A10 active cells. Finally, SR 48692 (10 mg/kg) modified neither accumbal nor cortical basal DA release. Local micro-injection of SR 48692 (10[-11]-10[-9] M), but not that of SR 49711 (10[-9] M), into the prefrontal cortex, increased the number of active A10 cells in a concentration-dependent manner. These results suggest that neurotensin receptor blockade counteracts a tonic inhibitory regulation by endogenous neurotensin of mesolimbic dopaminergic function and indicate that the prefrontal cortex is critically involved in this regulation.

Retrosplenial/presubicular continuum in primates: a developmental approach in fetal macaques using neurotensin and parvalbumin as markers

In spite of numerous hodological and neuropsychological studies emphasizing the multimodal connections and integrative functions of the retrosplenial cortex in primates, the precise fate of its caudoventral extent and the composition of the merging area with the hippocampal formation remain a matter of debate. We reported previously how the anlage of the retrosplenial cortex merges with the immature presubicular zone in the fetal rhesus monkey at the end of the first trimester of gestation. In the present study, this caudal area was further defined on a chemoarchitectonic basis, particularly during the late prenatal and perinatal stages, which correspond to the development of the cingulate sulcus and temporal gyri, and the differentiation of the retrosplenial/subicular complex. Neurotensin (NT), a pyramidal cell marker in the limbic cortex, and parvalbumin (PV), a marker of a subset of inhibitory local circuit neurons in the hippocampal formation, were used as immunocytochemical markers. According to distinct chemoarchitectural patterns, (1) areas 29 l and 29 m of the retrosplenial cortex formed a triangle-shaped ventral expansion which merged with a similar but dorsal expansion of the pre/parasubicular fields. A temporal extension of area 29 m down to area TH could not be detected. The pre/parasubiculum contributed with area 29 m to the lateral bank of the calcarine sulcus as far as the most caudal extent of the hippocampal formation. (2) The lamina principalis interna of the presubiculum was well individualized and did not appear as a simple horizontal shift of adjoining fields. (3) NT and PV displayed a distinct temporal profile of development. NT was already expressed in the pyramidal cells of the prospective retrosplenial cortex and ventral hippocampal formation at E47 (term 165 days). Major pathways of the hippocampal formation and retrosplenial cortex (fimbria, fornix, angular and cingulum bundles) were progressively labeled indicating early developing projections. A large set of NT-positive afferents reached the retrosplenial cortex between E114 and E120. Their laminar distribution was compatible with a thalamic or a subicular origin. (4) The development of PV expression was delayed until the last quarter of gestation, supporting its proposal as a signal of functional onset. The developmental fate and the particular connections of the presubiculum suggest that its functional importance should be further investigated during infancy and adulthood.

Opioids suppress spontaneous and NMDA-induced inhibitory postsynaptic currents in the dorsal raphe nucleus of the rat in vitro

Recently, local injection of morphine in the dorsal raphe nucleus (DRN) has been shown to increase serotonin release in the forebrain of unanesthetized rats. This study investigated the site of action of opioids in rat brain slices containing the DRN. Postsynaptic currents (PSCs), measured intracellularly under voltage clamp, were induced in serotonergic neurons with bath and microiontophoretic applications of NMDA to activate local neurons. Met-enkephalin (ENK) suppressed spontaneous and NMDA-induced GABAergic inhibitory PSCs. This effect, which was mimicked by the mu agonist DAMGO but not the kappa-agonist U50488 or the delta-agonist DPDPE, was reversed by the mu antagonist CTOP. ENK also suppressed spontaneous and NMDA-induced glutamatergic excitatory PSCs. By searching with focal microiontophoretic NMDA applications, GABAergic and glutamatergic cells projecting on serotonergic neurons were found in the DRN and the adjacent periaqueductal gray. Consistent with the reduction in PSCs, ENK inhibited/hyperpolarized the great majority (81%) of non-serotonergic neurons recorded extra- and intracellularly in the DRN; the ENK effect reversed polarity at -99 +/- 9 mV, close to the potassium reversal potential. In contrast, ENK inhibited/hyperpolarized only 28% of serotonergic neurons; in the affected cells, the ENK effect, blocked by CTOP, had its reversal potential shifted with change of extracellular potassium in agreement with the value predicted by the Nernst equation for a potassium conductance; serotonin occluded the ENK inhibition. Taken together, these results indicate that opioids inhibit both local GABAergic and glutamatergic cells projecting onto DRN serotonergic neurons.